The present invention relates to a reflective liquid crystal device. Such devices may be used, for instance, in hand-held and laptop equipment such as computers, diaries and personal organisers.
Proc. Ind. Acad. Sci., 1955, 41A 130 and 137 disclose circular polarisers and quarter wave plates made of combinations of birefringent plates to provide improved achromaticity. The individual retarders are combined with different azimuthal orientations of their optic axes to achieve the improvement in achromatic performance.
Seki et al, Mol. Cryst. Liq. Cryst., 1995, 263, 499 and Seki et al, Eurodisplay, 1996, 464 disclose a liquid crystal device (LCD) of the reflective electrically controlled birefringence (ECB) type comprising a nematic liquid crystal and a quarter wave plate. The optic axis of the quarter wave plate is crossed with that of the nematic liquid crystal and is at 45xc2x0 to a polariser disposed on one side of the nematic liquid crystal. The untwisted liquid crystal and quarter wave plate are disposed between the polariser and a mirror and a normally white state is achieved with the liquid crystal providing a retardation of xcex/4. A black state is achieved by controlling the liquid crystal so as to provide zero retardation. This black state is effectively provided by the quarter wave plate and is achromatic only to the degree that the quarter wave plate is achromatic. A further retarder of negative birefringence with its optic axis perpendicular to its plane may be included to improve viewing angle performance.
Uchida et al, Asia Display, 1995, 599 discloses a reflective display in which a polariser and a mirror are disposed on opposite sides of a hybrid aligned nematic (HAN) liquid crystal layer and a retarder. The retarder is biaxial having an optic axis out of the plane to improve viewing angle and an in-plane optic axis which co-operates with the retardation of the liquid crystal layer to provide black and white states. The retarder in-plane axis is at 45xc2x0 to the polarising axis of the polariser and is crossed with the optic axis of the liquid crystal layer. A normally black state is provided when the net retardation of the liquid crystal layer and the retarder is equal to xcex/4. A white state is provided when the retardation of the liquid crystal is such that it cancels the retardation of the retarder. Such a display depends for its achromaticity on optimising the dispersion of the optical elements.
Ishinabe et al, Eurodisplay, 1996, 119 discloses a full colour reflective LCD in which a HAN liquid crystal layer and a biaxial retarder are disposed between a linear polariser and a mirror. The optic axis of the HAN layer is crossed with the optic axis of the retarder and is at 45xc2x0 to the polarising axis of the polariser. A normally black state is achieved by making the difference between the retardations of the liquid crystal layer and the retarder equal to xcex/4. Achromaticity is improved by adjusting the dispersion of materials of the liquid crystal layer and the retarder so that the birefringences partially compensate each other.
Kuo et al, Asia Display, 1995, 135 also discloses an LCD in which a HAN liquid crystal layer and a biaxial retarder are disposed between a linear polariser and a mirror. A dark state is provided when the total retardation is equal to an odd number of quarter wavelengths. The retarder has an in-plane optic axis which is crossed with the liquid crystal axis and is at 45xc2x0 to the polariser axis. The display may be operated in the normally black mode, as described hereinbefore, or in the normally white mode, in which case a larger passive retarder is used. Achromaticity may be improved by optimising the dispersion of the elements or by adjusting the dispersion so that the birefringences compensate each other.
Wu et al, Applied Physics Letters, 1996, 68, 1455 discloses a reflective LCD in which a twisted nematic liquid crystal cell and a retarder are disposed between a polariser and a mirror. The twisted nematic liquid crystal cell has a twist angle of 90xc2x0, is relatively thin, and has its input director angled at 20xc2x0 to the axis of the polariser. The retarder provides a retardation of xcex/4 and has an optic axis angled at 45xc2x0 to the polariser axis. The cell operates in the normally white mode where the retardations of the liquid crystal cell and the retarder cancel each other in the white state and the black state is obtained by reducing the retardation of the liquid crystal cell to zero. Accordingly, the achromaticity depends on the achromaticity of the retarder.
Kuo et al, Eurodisplay, 1996, 387 discloses a similar twisted nematic display which is operated in the normally white mode and again achieves a black state whose achromaticity depends on the achromaticity of the retarder.
Fukuda et al disclose in three papers (IDRC, 1994, 201; SID Journal, 1995, 3, 83; Asia Display 1995, 881) a reflective supertwisted nematic (STN) LCD comprising a single polariser and a single retardation film. The twist of the liquid crystal is between 220 and 260xc2x0 and the device operates in the normally white mode. STN liquid crystal is used to allow high multiplex ratios i.e. small voltage differences between on and off voltages and hence a large value of dxcex94n of the liquid crystal, for instance greater than 0.6 micrometers. Achromaticity is improved by varying the dispersive properties of the liquid crystal and the retarder.
PCT WO 96/31577 discloses a reflective display comprising a linear polariser and a reflector between which are disposed a first retarder adjacent the polariser and a second retarder adjacent the reflector. The first retarder provides a retardation of xcex/2 and has an optic axis aligned at 15xc2x0 to the polarising direction of the polariser. The second retarder comprises a liquid crystal layer providing a retardation of xcex/4 with an optic axis which is switchable in the plane of the retarder between angles of 75xc2x0 and 120xc2x0 with respect to the polarising direction.
According to a first aspect of the invention, there is provided a reflective liquid crystal device comprising a linear polariser, a polarisation preserving reflector, and a retarder arrangement, characterised in that the retarder arrangement comprises at least three retarders, a first of which is disposed between the polariser and the reflector, a second of which is disposed between the first retarder and the reflector, and a third of which is disposed between the second retarder and the reflector, at least one of the first, second and third retarders comprising a liquid crystal layer which is switchable between a non-reflective device state, in which the retardation of the retarder arrangement is equal to (2n+1)xcex/4, where n is an integer and xcex is a wavelength of visible light, and a reflective device state.
In the non-reflective device state, the first retarder may have a retardation of substantially xcex/2, the second retarder may have a retardation of substantially xcex/2, and the third retarder may have a retardation of substantially xcex/4.
In the non-reflective device state, the optic axis of the first retarder may be substantially at an angle of (xcex1+B.180xc2x0) to the polarisation or absorption axis of the polariser, the optic axis of the second retarder may be substantially at an angle of (x.xcex1+C.180xc2x0) to the polarisation or absorption axis, and the optic axis of the third retarder may be substantially at an angle of (2(xcex2xe2x88x92xcex1)+sign(xcex1).45xc2x0+D.180xc2x0) to the polarisation or absorption axis, where x is a positive real number, B C and D are integers and sign (xcex1) if the sign of xcex1. xcex1 may be substantially equal to 6.90. x may be substantially equal to 5.
The first or second retarder may comprise the liquid crystal layer whose optic axis rotates during switching by an angle substantially equal to 22.5xc2x0 about the normal direction of light passage.
The second retarder may comprise the liquid crystal layer and may have a retardation which is switchable between substantially (pxcex/2+xcex4) and substantially ((p+1)xcex/2+xcex4), where p is an integer and 0xe2x89xa6xcex4 less than xcex/2. p may be equal to 0 or 1.
The third retarder may comprise the liquid crystal layer and may have a retardation which is switchable between substantially (qxcex/4+xcex4) and substantially ((q+1)xcex/4+xcex4), where q is an integer and 0xe2x89xa6xcex4 less than xcex/4. q may be equal to 0 or 1.
The liquid crystal layer may be an out-of-plane switching nematic liquid crystal.
In the non-reflective device state, the first retarder may have a retardation of 23xcex/72, the second retarder may have a retardation of xcex/2, and the third retarder may have a retardation of 23xcex/72.
In the non-reflective device state, the optic axis of the first retarder may be at an angle of substantially 14.25xc2x0 to the polarisation or absorption axis of the polariser, the optic axis of the second retarder may be at an angle of substantially 84.5xc2x0 to the polarisation or absorption axis, and the optic axis of the third retarder may be at an angle of substantially 14.25xc2x0 to the polarisation or absorption axis.
The second retarder may comprise the liquid crystal layer whose optic axis rotates during switching by an angle substantially equal to 22.5xc2x0 about the normal direction of light passage.
The second retarder may comprise the liquid crystal layer and may have a retardation which is switchable between substantially (rxcex/2+xcex4) and substantially ((r+1)xcex/2+xcex4), where r is in integer and 0xe2x89xa6xcex4 less than xcex/2. r may be equal to 0 or 1.
The third retarder may comprise the liquid crystal layer and may have a retardation which is switchable between substantially 23xcex/72 and substantially 23xcex/324 or substantially 46xcex/81.
The liquid crystal layer may be an out-of-plane switching nematic liquid crystal.
According to a second aspect of the invention, there is provided a reflective liquid crystal device comprising a linear polariser, a polarisation preserving reflector, and a retarder arrangement comprising a first retarder disposed between the polariser and the reflector and a second retarder disposed between the first retarder and the reflector, at least one of the first and second retarders comprising an untwisted liquid crystal layer which is switchable between a non-reflective device state, in which the retardation of the retarder arrangement is equal to (2n+)xcex/4, where n is an integer and xcex is a wavelength of visible light, and a reflective device state, characterised in that the first retarder comprises the liquid crystal layer and has an optic axis which rotates during switching by a predetermined angle about the normal direction of light passage.
In the non-reflective device state, the first retarder may have a retardation of substantially xcex/2 and the second retarder may have a retardation of substantially xcex/4.
In the non-reflective device state, the optic axis of the first retarder may be substantially at an angle of (xcex1+E.180xc2x0) to the polarisation or absorption axis of the polariser and the optic axis of the second retarder may be substantially at an angle of (2xcex1+sign(xcex1).45xc2x0+F.180xc2x0) to the polarisation or absorption axis, where E and F are integers and sign (a) is the sign of xcex1.
xcex1 may be substantially equal to 15xc2x0.
xcex1 may be substantially equal to 22.5xc2x0 and the predetermined angle may be substantially equal to 22.5xc2x0.
The liquid crystal layer may be a ferroelectric liquid crystal.
The liquid crystal layer may be an antiferroelectric liquid crystal.
The liquid crystal layer may be an electroclinic liquid crystal.
The liquid crystal layer may be an in-plane switching nematic liquid crystal.
According to a third aspect of the invention, there is provided a reflective liquid crystal device comprising a linear polariser, a polarisation preserving reflector, and a retarder arrangement comprising a first retarder disposed between the polariser and the reflector and a second retarder disposed between the first retarder and the reflector, at least one of the first and second retarders comprising an untwisted liquid crystal layer which is switchable between a non-reflective device state, in which the retardation of the retardation arrangement is equal to (2n+1)xcex/4, where n is an integer and xcex is a wavelength of visible light, and a reflective device state, characterised in that the liquid crystal layer is an out-of-plane switching liquid crystal.
In the non-reflective device state, the first retarder may have a retardation of substantially xcex/2 and the second retarder may have a retardation of substantially xcex/4.
In the non-reflective device state, the optic axis of the first retarder may be substantially at an angle of (xcex1+E.180xc2x0) to the polarisation or absorption axis of the polariser and the optic axis of the second retarder may be substantially at an angle of (2xcex1sign(xcex1).45xc2x0F.180xc2x0) to the polarisation or absorption axis, where E and F are integers and sign (xcex1) is the sign of xcex1.
xcex1 may be substantially equal to 15xc2x0.
The first retarder may comprise the liquid crystal layer having a retardation which is switchable between substantially (pxcex/2+xcex4) and substantially ((p+1)xcex/2+xcex4), where p is an integer and 0xe2x89xa6xcex4 less than xcex/2. p may be equal to 0 or 1.
The second retarder may comprise the liquid crystal layer and may have a retardation which is switchable between substantially (qxcex/4+xcex4) and substantially ((q+1)xe2x80x2/4+xcex4), where q is an integer and 0xe2x89xa6xcex4xcex/4. q may be equal to 0 or 1.
The liquid crystal layer may be an out-of-plane switching nematic liquid crystal.
According to a fourth aspect of the invention, there is provided a reflective liquid crystal device comprising a linear polariser, a polarisation preserving reflector, and a retarder arrangement comprising a first retarder disposed between the polariser and the reflector and a second retarder disposed between the first retarder and the reflector, at least one of the first and second retarders comprising a liquid crystal layer which is switchable between a non-reflective device state, in which the retardation of the retarder arrangement is equal to (2n+1)xcex/4, where n is an integer and xcex is a wavelength of visible light, and a reflective device state, characterised in that at least one of the first and second retarders comprises a twisted retarder.
The liquid crystal layer may comprise a homogeneously aligned nematic liquid crystal.
The liquid crystal layer may comprise a homeotropically aligned nematic liquid crystal.
The liquid crystal layer may be homogeneously aligned at a first surface and homeotropically aligned at a second surface.
The liquid crystal layer may be parallel-aligned.
The liquid crystal layer may be anti-parallel-aligned.
The retarder arrangement may comprise a further retarder whose optic axis is substantially perpendicular to the optic axis of the liquid crystal layer and which is disposed optically adjacent the liquid crystal layer.
The further retarder may have a retardation substantially equal to xcex4.
The further retarder may have a retardation substantially equal to the retardation of the liquid crystal layer in the absence of an applied field across the liquid crystal layer.
n may be equal to 0.
xcex may be between substantially 500 and substantially 570 nanometers. xcex may be between substantially 510 and substantially 550 nanometers. xcex may be between substantially 525 and substantially 530 nanometers.
The retarder arrangement may have a retardation substantially equal to mxcex/2 in the reflective device state, there m is an integer.
At least one of the first, second and third retarders may be a twisted retarder.
It is thus possible to provide an LCD which is suitable for use in reflective displays. High brightness and contrast are achieved in a reflective single polariser device which has a wide viewing angle and a fast response speed. For instance, it is possible to achieve an azimuthal viewing angle of greater than plus and minus 80xc2x0 without contrast inversion and in all azimuthal directions. It is further possible to achieve a black state having a degree of achromaticity which is substantially improved compared with known displays of the single polariser and reflector type. The optic axes of the retarders are oriented such that the combinations gives a retardation of xcex/4 for the input polarised light from the polariser over a wide range of wavelengths in the non-reflective or dark state. The dark state is therefore very dark and achromatic and this in turn gives high contrast when combined with the white state, which is also of high brightness. It is further possible to achieve a good achromatic reflective or white state. The retarders need not be made of the same material and the dispersive properties of the retarder materials are not substantially important for achieving achromaticity, although the best display is obtained by using identical materials of as low dispersion as possible.
Thus, a high brightness device is provided because of the use of a single polariser. Good achromatic behaviour permits high contrast to be achieved. The use of optically thin layers permits a wide viewing angle.
The present invention will be further described, by way of example, with reference to the accompanying drawings, in which: